To improve the adhesive properties of synthetic filaments towards rubber, it is customary to treat them with a composition which is a mixture of latex and resorcinol-formaldehyde (RFL) resin. This composition is then hardened on the filaments by means of a thermal treatment.
The most widely used synthetic filaments for reinforcing rubber-based products, such as automobile tires, flexible tubes, V-belts and conveyor belts are polyester filaments. There is, however, a serious disadvantage associated with polyester reinforcing filaments. This disadvantage is that such filaments have an insufficient number of reactive end groups, thus resulting in a poor filament/RFL bond.
In view of the above, the industry has discovered that it is necessary to subject the polyester filaments to a pretreatment process. This pretreatment, also referred to as preadherization, is very often carried out by the fiber manufacturer himself, so that the consumer can carry out the adherization with RFL before vulcanizing the filament with rubber.
There are numerous existing processes for preadhering polyester filaments. U.S. Pat. Nos. 3,383,242 and 3,297,468, for example, relate to a process for pretreating polyethylene terephthalate filaments by applying to said filaments: (a) a dispersion of a hardenable combination of a diepoxide and a diamine having primary or secondary functions (hardening agent) and (b) a water-dispersible filament finish. The application process disclosed therein takes place on an undrawn filament, which is wound up and subsequently conventionally stretched in a second operation.
The disadvantage of the preadhering process disclosed therein is that it is necessary to use a composition which solidifies into a gel-like mass after a few hours at room temperature. The stability of such a solution is thus poor.
Moreover, the filament cannot be drawn using heated rollers, but can only be drawn in ovens, even at slow take-off speeds. In view of this, excess preadhering solution would harden on the rollers and would quickly build up into a crust. It has also been observed that the filaments pretreated by the process disclosed therein have impaired mechanical properties.
In U.S. Pat. No. 3,962,011 nylon cord yarns (i.e. already plied yarns), are impregnated with a combination of a polyepoxide and a tertiary amine. The combination is then thermally hardened.
U.S. Pat. No. 3,793,425 and German Offenlegungsschrift 2,056,707 relate to an integrated spin-drawing process for producing polyester filament yarns by applying a composition containing a polyepoxide resin buffered at an alkaline pH to the freshly spun and likewise still unstretched polyester filament. After the composition has been applied thereto, the treated filaments are then hot-stretched and wound-up.
U.S. Pat. No. 3,464,878 describes a system composed of polyepoxide and a pyrrolidone which is catalyzed with an alkaline compound (isocyanate, NaOH). Due to a curing process, this catalyst releases a tertiary amine. The concentration of the pyrrolidone and the catalyst are extremely high. The process of U.S. Pat. No. 3,464,878 results in a subcoated fiber of about 0.3% by weight. Moreover, this subcoat is cured (i.e., under conditions of heating to 218.degree. C. for one minute). Although tertiary amines are being released, U.S. Pat. No. 4,464,878 does not disclose the use of these tertiary amines.